Presynaptic mechanisms are an important means for neurotransmitters, hormones, paracrine substances and drugs to modulate the activity of myenteric nerves and gut motility. Despite the clear importance of presynaptic mechanisms in regulating neurotransmission in the myenteric plexus, little is known about the details of these mechanisms. The proposed studies are designed to characterize these mechanisms. Electrophysiological methods will be used to study excitatory synaptic transmission between myenteric neurons in acutely isolated preparations of guinea pig intestine and between guinea pig myenteric neurons maintained under tissue culture conditions. These studies will investigate the contributions of acetylcholine (ACh) acting at nicotinic cholinergic receptors and ATP acting at P2X purine receptors, to fast excitatory synaptic potentials (fEPSPs) during long trains of stimulation. As nicotinic receptors desensitize quickly, it is anticipated that ACh will be the predominate transmitter early while ATP will predominate later in the train of activity. Presynaptic mechanisms controlling the release of these two important transmitters will also be investigated. These studies will focus on channels which provide the calcium needed for transmitter release and the presynaptic receptors which facilitate their release. The facilitatory effects of 5-HT4 receptor activation on fEPSPs mediated by ACh and ATP will be investigated. In addition, these studies will investigate the contribution of 5-HT3 and 5-HT4 receptors to excitation of enteric sensory nerve terminals. The role of presynaptic nicotinic acetylcholine receptors (nAChRs) in controlling the release of the slow synaptic transmitters, substance P/neurokinin A will also be investigated. These latter studies will attempt to establish that there are presynaptic nAChRs on the terminals of sensory neurons and that ACh released from sensory neurons facilitates SP/NKA release and enhances slow excitatory synaptic transmission. There are many disorders of gut motility which have their basis either directly or indirectly in alterations in enteric neurotransmission. These disorders include gastroesophageal reflux disease, the irritable bowel syndrome and chronic intestinal pseudo-obstruction. It is anticipated that these studies will provide new insights into mechanisms of synaptic transmission in the gut which could lead to the development of new and effect drug treatments for motility disorders.